Origin of the 0.25 anomaly in the nonlinear conductance of a quantum point contact

We calculate the nonlinear conductance of a quantum point contact using the nonequilibrium Green's function technique within the Hartree approximation of spinless electrons. We quantitatively reproduce the 0.25 anomaly in the differential conductance (i. e., the lowest plateau at similar to 0.25-0.3 x 2e(2)/h) as well as an upward bending of higher conductance half-integer plateaus seen in the experiments, and relate these features to the nonlinear screening and pinning effects.